It is of great importance to be able to detect sub-surface vessels such as submarines. One detection method includes monitoring changes in the magnetic field of a given area. That is, as a submarine moves, it alters the magnitude and direction of Earth's natural geomagnetic field. Although these changes to the geomagnetic field are very small, they are detectable by high precision magnetometers. The use of magnetometers to detect enemy vessels in Anti-Submarine Warfare (ASW) operations is known as Magnetic Anomaly Detection (MAD), and this is a technique that has been widely used by the world's navies since the days of World War II.
MAD for ASW is usually performed by mounting a high precision magnetometer on the tail of an aircraft, helicopter, or Unmanned Aerial Vehicle (UAV). The aircraft traverses an area of interest measuring the magnetic field. These measurements are compared to established geomagnetic records. Any magnetic deviation to the established geomagnetic field of the area is then interpreted as the potential presence of an adversary submarine.
Alternatively, MAD for harbor protection is accomplished by mounting magnetometers on buoys that surround the area of interest. These buoys float on the surface and transmit their measurements to some location that performs data fusion to determine if enemy vessels are disturbing the geomagnetic field of the area.
Even though ASW and harbor protection are defensive operations, MAD can be used for offensive operations such as mine warfare. In this case, magnetometers are installed on the mines, and a given mine is triggered if its associated sensor detects substantial deviations to the geomagnetic field.
The vast majority of ASW operations are aircraft based. However, there are several shortcomings to the use of MAD sensors for aircraft mounted ASW operations:                i. As the magnetometer is usually mounted on the tail of an aircraft, it is subject to a large amount of noise due to the magnetic anomaly produced by the aircraft itself.        ii. This approach only works when the distance between the magnetometer/aircraft and the submarine is not too large (the aircraft has to fly at very low altitude and the target submarine has to travel at a shallow depth). However, most of the threat to, e.g., an aircraft carrier group, comes from submarines operating at substantial depths.        iii. MAD over an extended area of interest necessitates that the aircraft fly a swathing path that covers a large grid. This technique does not guarantee a successful detection of the submarine, as the enemy vessel may be fortunate to traverse the area when the aircraft is in a different region.        iv. Each littoral area has its own, often time-dependent, magnetic field topography defined by naturally occurring metallic materials, along with man-made metallic materials such as port infrastructure and sunken debris. As such, it is difficult to keep an up to date geomagnetic map of the entire globe.        v. The magnetic field surrounding a carrier group is complex, and imposes a challenge to determine the unperturbed magnetic configuration to establish the presence of an enemy vessel.        
Some disadvantages for the use of MAD for harbor protection include the fact that buoys are big, bulky, and relatively expensive. Therefore, it is extremely difficult, time consuming, and expensive to deploy them. In the case where an operation has to be mounted in an obscure part of the world, the use of big buoys is not an efficient solution.
Finally, some common disadvantages for the use of MAD for ASW, harbor defense, and mine warfare are:                i. High precision magnetometers are big and bulky.        ii. High precision magnetometers are expensive.        iii. High precision magnetometers have considerable power requirements.        iv. Some high precision magnetometers have operational restrictions regarding temperature and humidity.        v. High precision magnetometers are difficult to ruggedize for the undersea combat environment.        vi. High precision magnetometers are complex to mass produce on a timely manner.        vii. Most of the highest precision magnetometers available are scalar detectors, in the sense that they can detect the magnitude of the magnetic field, but not its direction.        viii. Some of the highest precision magnetometers currently available, such as those based on Superconducting Quantum Interference Devices (SQUID), require cryogenic needs. As such, their ruggedization for the battlefield environment is extremely difficult.        
In light of the foregoing, there is a need for a different approach to ASW operations, among other types of operations.